Vanesa Ruiz

A homologous series of ionic liquids (IL) with 1-alkyl-3-methylimidazolium cations of different lengths of alkyl chain was used to study the effect of cation size on the capacitive response of two carbons with a tailored pore size distribution. The results reveal a clear ion-sieving effect in pure ILs, while the effect is heavily mitigated for the same salts used in solution, most likely due to somewhat stronger geometrical flexibility of dissolved ions. For the electrode material showing the ion-sieving effect in solution, the gravimetric capacitance values are higher than in pure ILs. The dissimilarity of capacitance values between pure and dissolved ILs with ion-sieving carbons highlights their respective advantages and disadvantages in terms of energy density: whereas pure ILs can potentially provide a larger working voltage window, the corresponding dissolved salts can access smaller pores, mostly contributing to higher capacitance values.

The present work reports the use and application of a novel protic ionic liquid (triethylammonium bis(trifluoromethylsulfonyl)imide; NEt3H TFSI) as an electrolyte for symmetric planar micro-supercapacitors based on silicon nanowire electrodes. The excellent performance of the device has been successfully demonstrated using cyclic voltammetry, galvanostatic charge-discharge cycles and electrochemical impedance spectroscopy. The electrochemical characterization of this system exhibits a wide operative voltage of 4 V as well as an outstanding long cycling stability after millions of galvanostatic cycles at a high current density of 2 mA cm−2. In addition, the electrochemical double layer micro-supercapacitor was able to deliver a high power density of 4 mW cm−2 in a very short time pulses (a few ms). Our results could be of interest to develop prospective on-chip micro-supercapacitors using protic ionic liquids as electrolytes with high performance in terms of power and energy densities

In this paper, phosphomolybdic acid H3PMo12O40 (PMo12) was anchored to four synthetic micro-mesoporous carbons and a commercial one to analyse the relationship between the porous texture of the support, the PMo12 adsorption and the performance of the resulting hybrid materials as electrodes in supercapacitors. The uptake of PMo12 on carbon supports follows a clear correlation with the micropore volume, which implies that PMo12 is mainly adsorbed in microporosity as a consequence of a greater confinement in this kind of pores instead of mesopores. Transmission electron microscopy indicates that the PMo12 adsorbed is homogeneously dispersed in the carbon texture. Finally, the addition of PMo12 to the original carbon electrodes provided capacitances up to 293 F per gram of electrode, substantially larger than the 206–240 F g−1 of the unmodified activated carbon. This result represented an increase of up to 35% in terms of gravimetric energy density and 160% in terms of volumetric energy density, after PMo12 integration into the carbon matrix.

Stable anchoring of polyoxometalate H3PMo12O40 onto in-situ Reduced Graphene Oxide leads to a hybrid electrode with combined capacitive (GO) and faradaic (H3PMo12O40) charge storage mechanisms featuring increased capacitance, energy density and extended cyclability.

The development of a novel hybrid symmetric micro-ultracapacitor based on poly(3,4-ethylenedioxythiophene) coated silicon nanowires using an ionic liquid (N-methyl-N-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide) as an electrolyte has been demonstrated. The hybrid supercapacitor device was able to deliver a specific energy of 10 Wh kg-1 and a maximal power density of 85 kW kg-1 at a cell voltage of 1.5 V. The hybrid device exhibited long lifetime and an outstanding electrochemical stability retaining 80 % of the initial capacitance after thousands of galvanostatic charge-discharge cycles at a high current density of 1 mA cm-2. The improvement of the capacitive properties compared with the bare SiNWs was attributed to the pseudo-capacitive behavior induced by the conducting polymer coating.

This work describes the development and performance of a symmetric micro-supercapacitor made of nanostructured electrodes based on silicon nanowires (SiNWs) deposited using chemical vapor deposition (CVD) on silicon substrates. The performance of the SiNWs micro-supercapacitor employing an aprotic ionic liquid (N-methyl-N-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide) (PYR13TFSI) as an electrolyte was able to deliver a maximal power density of 182 mW cm−2 and a specific energy of 190 μJ cm−2 operating at a wide cell voltage of 4 V with a quasi-ideal capacitive behavior. The lifetime of the device exhibited a remarkable electrochemical stability retaining 75% of the initial capacitance after several million galvanostatic charge–discharge cycles at a high current density of 1 mA cm−2. Furthermore, a coulombic efficiency of approximately 99% was obtained after galvanostatic cycling test without structural degradation on the morphology of SiNWs.

The hybrid approach allows for a reinforcing combination of properties of dissimilar components in synergic combinations. From hybrid materials to hybrid nanocomposite electrodes the approach offers opportunities to tackle much needed improvements in the performance of energy storage devices. This paper presents different types of hybrid materials with emphasis on those synthesized and studied in our laboratory and reviews the different approaches and scales of hybrids, materials, electrodes and finally presents recent efforts to develop hybrid battery-supercapacitor devices

In order to understand the participation of electrical double layer and pseudocapacitance to the overall behavior of supercapacitors, a new approach to the analysis of the electrochemical data is proposed. Both the variation of the specific capacitance values and the dependence of these values with the operating voltage window (varying from 0-0.2 V to 0-1 V) were evaluated and used to quantify the contribution arising from each mechanism of energy storage to the total capacitance of the system.

The suitability of the methodology here proposed was tested in various carbon materials (multiwalled carbon nanotubes, a carbon aerogel and two activated carbons), different both in nature and physicochemical characteristics. For all of the carbons studied, the capacitance with an exclusive faradic and non-faradic origin was quantified. Whereas some of the carbons studied showed a behavior close to an ideal electrical double layer capacitor (EDLC) with virtually no pseudocapacitance contribution (case of the carbon nanotubes), others presented up to a 40% of pseudocapacitance contribution (case of KOH-activated carbon

The design and development of hybrid organic- inorganic materials with improved electrochemical performance to be used in energy storage applications (e.g. supercapacitors) has been investigated. The systematic study carried out shows carbon–based materials where the anchoring of inorganic species takes place (i.e. polyoxometalates, POM) in search of synergic properties. In this paper, a Kegging-type polyoxometalate, the phosphomolybdic acid H3PMo12O40 (PMo12), was anchored to an activated carbon. The specific capacitance values obtained from the cyclic voltammogram in a 3-electrode configuration for the pristine activated carbon and the related hybrid material are 185 Fg-1 and 246 Fg-1 respectively (at 10 mVs-1)

Hybrid electroactive materials based on Polyoxometalate – adsorbed Activated Carbons have been developed for the first time as electrodes for Supercapacitors. By combining the double layer capacitance of a high–surface area material with the reversible redox activity of a polyoxometalate (POM), enhanced electrochemical performance is obtained.

In this paper, phosphomolybdic acid H3PMo12O40 (PMo12) was anchored to an activated carbon providing 160 Fg- 1 and 183 Fg- 1 for the positive and negative hybrid electrodes, respectively (at 2 Ag- 1), substantially larger than the 136 Fg- 1 of the unmodified activated carbon. This translates to a cell capacitance for the AC–POM supercapacitor is 45 Fg- 1 versus 35 Fg- 1 for the AC supercapacitor, i.e. a 29% increase after POM integration into the AC matrix.

Ionic liquid (IL)-based electrolytes containing molecular solvents were shown to be attractive for extreme temperature applications in electric double layer capacitors (EDLCs). In particular, the IL–butyronitrile (BuCN) mixture provides high capacitance (around 125 F g−1 at 500 mA g−1) independent of testing temperature, and superior performance at high current rates (reduced current dependence at high rates). Importantly, the IL–BuCN electrolyte can safely operate between −20 and + 80 °C, which overcomes the high temperature limitations of current commercial EDLCs. An additional advantage of IL–solvent mixtures is that the higher concentration of IL ions in the mixtures allows a greater specific capacitance (F g−1) to be achieved. The conductivity of the ionic liquid N-butyl-n-methylpyrrolidinium bis(trifluoromethane sulfonyl) imide (PYR14TFSI) could be increased from 2.48 mS cm−1 up to 45 mS cm−1 by mixing with an appropriate solvent. Importantly, these solvent mixtures also retain a wide electrochemical voltage window, in the range 4–6 V.

Porous carbons with controllable and narrow pore-size distributions are prepared from the chemical activation of polyfurfuryl alcohol (PFA). High apparent BET surface areas, up to 2600 m2 g−1 (2611 m2 g−1 by Density Functional Theory (DFT)), and good electrical conductivities (up to ∼130 S cm−1) are obtained. By varying the potassium hydroxide: carbon precursor ratio, the preparation of carbons with different proportions of micro- and fine mesoporosity (<5 nm) can be tailored to provide an ideal electronic and ionic pore structure for electrochemical energy-storage devices, such as electrical double-layer capacitors. High specific capacitance values are obtained up to 147 F g−1 in a voltage window of 2.5 V using 1 M tetraethyl ammonium tetrafluoroborate in acetonitrile. Moreover, excellent high-current and high-frequency performance is demonstrated: 100 F g−1 at 225 A g−1 (10 Hz) and ∼30 F g−1 at 100 Hz. When comparing the performance with commercial activated carbons (ACs) of similar textural properties, the PFA-derived ACs demonstrated better performance in terms of higher capacitance values and improved rate capabilities. There is a 125% increase in capacitance values at 1 kHz.

A pyrolysis product derived from a coal-tar pitch was chemically activated using KOH in a KOH/carbon proportion of 5:1. The activated carbon was used as electrode-active material for capacitive deionization (CDI). Electrochemical parameters, such as current values, charge, specific charge and charge-discharge efficiencies were investigated using a unit cell and solutions of NaCl in different concentrations. The parent activated carbon shows an excellent behavior as electrode-active material in CDI, removing more salt than other carbons previously described in the literature. The activated carbon electrode presents an efficiency higher than 99% after 20 cycles. The parent activated carbon was modified by thermal treatment under nitrogen at different temperatures and by treatment with hydrogen and carbon dioxide. The modified activated carbons were also evaluated as electrode-active material to study the influence of the texture and surface chemistry on the CDI process. The results show the importance of both the texture and surface chemistry of the active material on the CDI process. The best behavior as electrode-active material was obtained for the materials with a high apparent specific surface area and a large quantity of-oxygenated functional groups, i.e., the parent activated carbon and the sample modified by hydrogen treatment.

Polyfurfuryl alcohol (PFA) derived activated carbons were prepared by the acid catalysed polymerization of furfuryl alcohol, followed by potassium hydroxide activation. Activated carbons with apparent BET surface areas ranging from 1070 to 2600 m2 g−1, and corresponding average micropore sizes between 0.6 and 1.6 nm were obtained. The porosity of these carbons can be carefully controlled during activation and their performance as electrode materials in electric double layer capacitors (EDLCs) in a non-aqueous electrolyte (1 M Et4NBF4/ACN) is investigated.

Carbon materials with a low average pore size (<∼0.6 nm) exhibited electrolyte accessibility issues and an associated decrease in capacitance at high charging rates. PFA carbons with larger average pore sizes exhibited greatly improved performance, with specific electrode capacitances of 150 F g−1 at an operating voltage window of 0–2.5 V; which corresponds to 32 Wh kg−1 and 38 kW kg−1 on an active material basis. These carbons also displayed an outstanding performance at high current densities delivering up to 100 F g−1 at current densities as high as 250 A g−1. The exceptionally high capacitance and power of this electrode material is attributed to its good electronic conductivity and a highly effective combination of micro- and fine mesoporosity.

The capacity to store energy in ultracapacitors or double-layer capacitors is about two orders of magnitude greater than that of ordinary capacitors. Also, their capability to receive and transfer power is greater than that of batteries in the same order of magnitude. These characteristics, together with their long life cycle ( $>$ 500 000) and their lack of maintenance work, make them a suitable choice in replacing batteries in pulsed or discontinued power consumption systems. This paper tries to evaluate the possibility to replace batteries with ultracapacitors in Global System for Mobile Communications (GSM)-based remote supervision systems using photovoltaic panels as main power source.

The behaviour of mesophase-derived electrodes on long-duration cycling conditions was studied in 1 M sulfuric acid and 6 M potassium hydroxide. Variation in the specific capacitance values with the number of cycles shows a good cycling life performance in acidic media but very poor in an alkaline electrolyte. The total loss of capacitance after 7000 cycles in acidic media is 8% at 0.6 V and 16% at 1 V, whereas in the basic electrolyte the reduction in the capacitance values is 72%, even at a very small operating voltage (0.6 V). This behaviour was associated to the strong oxidation of the positive electrode caused by the progressive shift of the working potential towards very positive potential values during cycling.

This paper studies the effect of thermal treatment on the long-term performance of carbon-based supercapacitors. Two active electrode materials were studied: a mesophase derived activated carbon (AC), rich in oxygen functionalities, and an activated carbon obtained from treating AC at 1000 °C (AC-1000), from which most of the functionalities had been removed. Up to 10,000 cycles were performed in aqueous acidic media at different voltage windows (0.6 and 1 V). Both materials showed an excellent life cycle, although the thermally treated carbon showed a significantly better behavior. The initial capacitance of AC-1000 was reduced by only ∼5% after 10,000 cycles, independently of the operating voltage, demonstrating that the thermal treatment of AC produces a very stable electrode material at both ranges of potential. The better performance of AC-1000 was attributed to the absence of functional groups and the higher degree of crystalline order that renders materials more stable. The use of these two materials in an asymmetric device resulted in a capacitor that merges both high stability and high capacitance values. The initial capacitance was reduced by only 4.5% after 10,000 galvanostatic cycles and by 10% after 20,000.

This paper reports on the electrochemical performance of carbon-based electrodes subjected to thermal treatment. Changes in texture and chemical composition were evaluated and used to assess the effect of thermal treatment on the electrochemical performance of the resultant supercapacitors. Disc-type electrodes were prepared using polyvinylidene fluoride, PVDF, as binder and a mesophase-derived activated carbon as active material. The carbonization of the electrodes under inert atmosphere (N2-flow) at 600 and 1000 °C leads to the decomposition of the polymer making it possible to recover most of the surface area that is blocked by the polymer. The carbonized electrodes were tested as electrodes in supercapacitors, with the result that lower specific capacitance values were obtained than with the “un-treated” electrodes. This was attributed to the formation of constrictions at the entrance of the porous network as a consequence of the polymeric decomposition. Furthermore, electrical conductivity, power density and long-cycling performance showed a significant improvement as a result of the carbonization procedure.

This work investigates the influence of electrode preparation on the electrochemical behaviour of carbon-based supercapacitors. Studies were performed using the same activated carbon and polymer polyvynilidene fluoride (PVDF) in the same proportions (10 wt.% PVDF). Only the way in which these components were mixed was modified. The procedure for mixing the activated carbon and the polymer has a significant influence on the electrochemical behaviour of the electrode used in a supercapacitor, as this determines the surface area accessible to the electrolyte. The mixing procedure can be selected in order to ensure optimum performance of the electrode. The use of N-methyl-2-pyrrolidone (NMP) in the mixing procedure, the most common method reported in the literature, blocks a significant part of the porosity of the activated carbon, causing a decrease in capacitance. The addition of the polymer using one of the other methods studied reduces the accessible surface area to a lesser extent, although the use of ball milling causes a decrease in the size of the carbon particles which, in turn, increases the electrode resistance.

This paper studies the electrochemical behaviour of activated carbons with different oxygen content and investigates the contribution of pseudocapacitance to the global behaviour of the samples. A mesophase-derived activated carbon was further heat treated to 600 or 1000 °C in nitrogen. The changes in texture and surface chemistry induced by the thermal treatment were deeply studied. The electrochemical behaviour of the samples was studied in two- and three-electrode cells. The contribution of pseudocapacitance was evaluated by cyclic voltammetry and by the differences of specific capacitance obtained from galvanostatic tests performed in acidic (H2SO4) and basic (KOH) media. The presence of an extra capacitance due to redox reactions has been proved both in acidic and basic media for the samples with high oxygen content, although its contribution in basic media is significantly lower. The results obtained clearly indicate that the oxygen responsible for CO-evolution participates in redox reactions, whereas the oxygen responsible for the CO2-evolution is of minor importance.

Mesophase pitch AR24 was directly activated with KOH using different proportions of the activating agent and activation temperatures, to study the effect on the textural characteristics of the resultant activated carbons and how these characteristics influence their behaviour as electrodes in supercapacitors. The textural properties of the activated carbons were studied by gas adsorption and immersion calorimetry. The results indicate that all the carbons produced were mainly microporous, with pore size around 1 nm. The behaviour of these carbons as electrodes in supercapacitors was studied from galvanostatic charge–discharge cycles. The specific capacitance values obtained were very high, reaching 400 and 200 F g−1 at low and high current densities respectively, for the sample activated with (5:1) KOH to mesophase ratio. Nevertheless, the reasons for this high capacitance values cannot be explained only on the basis of the textural characteristics of the activated carbons, as the results indicated that other factors might be also playing a significant role in their electrochemical behaviour.

A pyrolysis product derived from Sasol-Lurgi gasifier pitch was activated using different proportions of KOH. The increase of the amount of KOH used for activation caused the activation degree of the carbons to increase very significantly. The activated carbons obtained using lower amounts of KOH were mainly microporous, while the amount of mesopores developed in the samples progressively increased for the carbons activated with higher proportions of KOH. The gravimetric specific capacitance of samples obtained with (2:1), (3:1) and (5:1) KOH to carbon ratio were rather similar at low current densities (∼400 F/g at low current densities), despite the significant differences observed in their textural characteristics. Supercapacitors built with the activated carbons obtained with (2:1) and (3:1) KOH to carbon ratio yielded the highest volumetric capacitance (higher than 200 F/cm3 at low current densities), while the most activated sample yielded the lowest values, due to the significant reduction in density caused by activation. The high values of capacitance observed result from the combination of two mechanisms of energy storage: double layer formation and pseudocapacitance.